Write a unit test to trigger the error code path in fork_dup_all_handles()

[vvaltchev]

As it can be seen here: https://app.codecov.io/gh/vvaltchev/tilck/blob/master/kernel/fork.c, currently the following code path inside fork_dup_all_handles() is not covered by tests:

      if (rc < 0 || !dup_h) {

         enable_preemption();
         {
            for (u32 j = 0; j < i; j++)
               vfs_close(pi->handles[j]);
         }
         disable_preemption();
         return -ENOMEM;
      }

Now that we have gMock in Tilck, it would be relatively simple to just mock vfs_dup and vfs_close() so that inside a unit test we can trigger this code path. To do that, it would be necessary to make the fork_dup_all_handles() function STATIC instead of static, so that in unit tests, the function won't be static anymore. Also, it would be necessary add the mocked functions to a CMakeLists.txt file and to another file in the unit tests directory. For the exact instructions, see the comments in: https://github.com/vvaltchev/tilck/blob/master/tests/unit/mock_experiments.cpp

Clearly, because the fork_dup_all_handles() function takes a struct process * parameter, the unit test will have to allocate such a structure and set at least 2 handles in the pi->handles array.

[michelececcacci]

Even though i'm not a first time contributor i would like to take this.

[vvaltchev]

@michelececcacci Sure, take the issue. You haven't contributed in this area, so I guess it's still a very good first task for you.